Metal cleaning process and composition



Patented June 12, 1945 UNITED STATES PATENT OFFICE METAL CLEANING PROCESS AND COMPOSITION No Drawing. Application April 23, 1941. Serial No. 389,919

12 Claims.

This invention relates to the art of cleaning metals and more particularly to a method for cleaning metals in fused baths.

Oxide scale and other surface impurities are commonly removed from steel and other metal articles by treatments in aqueous solutions of acids or alkalies, with and without the utilization of electric current. A great variety of such cleaning solutions and processes are known and used for treating different kinds of metal articles and for different purposes. In some cases difliculties are encountered because of the chemically resistant nature of the surface impurities to be removed and in such case it is often necessary to use drastic chemical treatments which not only remove the surface impurities but also dissolve a considerable portion of the underlying metal. This difficulty is encountered for example in the treatment of certain chrome alloy steels. In order to properly clean such steels for certain purposes and to remove mill scale, and the like therefrom. itis often necessary to use a drastic treatment whichremoves a considerable portion of the alloy steel surface. Because of the expensive nature of many of these alloy steels a process has been desired which would readily remove the mill scale and other surface impurities without substantial attack on the underlying metal surface.

An object of the present invention is to provide 3 an improved method for cleaning metal surfaces. particularly a method which will adequately remove surface impurities without substantial attack on the underlying metal. A further object is to provide a fused alkali bath which is suitable for cleaning steel articles without substantial attack on the steel. Further object will be apparent from the following description of my invention.

I have discovered that the above objects may be attained by contacting metals with a fused bath composed of molten alkali metal hydroxide having dissolved therein an alkali metal hydride. At the temperatures common in such fused baths. for example from 350-500" C., I have found that the dissolved alkali metal hydride very rap dly reacts with the oxide coatings and commonly occurring surface impurities on steel and other metal articles, while there is no indication of any appreciable attack on the underlying metal.

In on method of practicing my invention I may first provide a fused bath comprising an alkali metal hydroxide containing in solution an appreciable quantity of an alkali metal hydride.

I may for example utilize a bath consisting of 55 molten caustic soda having dissolved therein from 1-20% of sodium hydride (NaH). Preferably the bath is protected from oxidizing influences so as to prevent the formation of alkali metal oxide in the bath, although in some cases small amounts of alkali metal oxide are not detrimental. A preferred method of preventing oxidation of the active ingredient of the bath is to continuously pass a stream of hydrogen gas into the bath. The hydrogen may or may not be preheated, as desired.

A convenient and preferred method of operating my process consists in floating a layer of molten sodium on the surface of a fused caustic soda bath and passing in hydrogen so that it comes into contact with the sodium at the interface between the sodium and caustic layers, for example by passing the hydrogen in below the surface of the caustic so that, it rises through the caustic and thence into the layer of sodium.

In this way the sodium hydride which is 'formed in the presence of the molten caustic soda immediately dissolves therein, forming a solution of the hydride in the caustic soda. The layer of sodium metal protects the hydride in the bath from oxidation with the air and the sodium layer may in turn be protected from oxidation if desired. for example. by maintaining over it an atmosphere of non-oxidizing gas such as hydrogen. hydrocarbon, or nitrogen. If desired, by the means of a suitable partition a small portion of the bath wherein work is to be introduced may be kept free from the sodium layer and if desired, this portion of the bath may be protected from the air by means of a non-oxidizing gas atmosphere or by the use of a floating layer of solid material insoluble in the bath, for example finely divided carbon such as charcoal or graphite. When operating with relatively low hydride concentrations in the bath, e. g, l to 5% by weight, it is not essential that the bath surface be so protected.

In another method of preparing the herein described sodium hydride solution, I may react h.v drogen with a fused mixture of alkali metal oxide and alkali metal hydroxide. for example, a mixture of sodium monoxide and sodium hydroxide. Such alkali metal oxide readily dissolves in alkali metal hydroxide and in that solution it readily reacts with hydrogen to form the hydride. In operating a bath prepared in this manner it is usually preferable to convert all of the alkali metal oxide to hydride before treat ng the work therein. When the alkali metal hydride content becomes depleted some of the bath may be regenerated by adding more alkali metal oxide and again treating the bath with hydrogen. Preferably, the treatment with hydrogen may be continued while the bath is being utilized to clean metal articles, in order to insure complete reduction of oxides at all times.

Baths suitable for my cleaning process also may be made by dissolving preformed solid sodium hydride or other alkali metal hydride in the alkali metal hydroxide fused bath.

The invention is not restricted to baths which contain only-alkali metal hydroxide and hydride, as various fusible diluents may be added without departing from the spirit and scope of my invention. For example alkali metal halides, carbonates or other fusible inorganic compounds suitable article converting the'hydride to alkali metal oxide and alkali metal peroxide, which latter compound tends to cause reoxidation of the cleaned for addition to fused salt heat treating baths may be added, including fusible compounds of metals other than the alkali metals. Addition of such compounds may be made in order to obtain baths having various melting points. For example, by addition of sodium iodide to a caustic soda bath containing sodium hydride compositions may be made having low melting points, so that the baths may be operated at temperatures as low as about 250 C. Similar low melting compositions may be made by dissolving the alkali metal hydride in fused mixtures of sodium and potassium hydroxides. The various diluents which can thus be added to vary the melting point of the bath will be apparent to those skilled in compounding fused baths for metal treatment. Obviously, any

ingredient added to the baths must be substantially chemically inert to the alkali metal hydroxide and hydride or at least if it combines with the hydroxide must form a fusible compound which likewise will dissolve the alkali metal hydride. Also, all bath ingredients must be substantially non-oxidizing in character.

While I prefer to use sodium hydride and sodium hydroxide because of their cheapness and availability, the hydrides and hydroxides of the other alkali metals, e. g., potassium, and lithium, may be used with substantially equal effectiveness.

It is essential in the practice of my invention that the alkali metal hydride be dissolved in .the fused bath. I have discovered that whereas alkali metal hydrides in the free state readily decompose when heated at temperatures in the range of 300- 600 0., when dissolved in a molten composition such as one containing alkali metal hydroxide, the hydrides have a remarkably high thermal stability and undergo little or no decomposition at these temperatures in the absence of oxides or other oxidizing agents. At the same time, the dissolved hydride is chemically very reactive and readily attacks metal oxides to cause their reduction.

In most cases after treatment in the fused hydride bath, it is preferable to immediately quench the treated metal articles with water, in order to obtain the best cleaning results. For example I have found that in cleaning certain types of chrome alloy steels the hydride bath treatment alone does not completely remove mill scale but reduces it to form a. slightly adherent coating on the steel. When the steel articles are removed from the hydride bath and quenched with water the reduced scale and other impurities immediately separate from the steel, leaving a clean, bright surface. The quenching with water may be accomplished either by dipping the article in a bath of water or by spraying or otherwise flowing a stream of water on to the article as desired. The water quenching also serves to prevent remetal surface. For this reason it is preferable to remove the adherent film of the bath from the articles immediately after their removal from the bath. While this may be done by mechanical means for example by wiping, it is more convenient and preferable in most cases to simply immerse the treated article in water or otherwise subject it to a washing operation in an aqueous medium.

The bath temperature for the metal cleaning operations may be varied as desired over a wide range above the melting point of the molten bath. For example, using solutions of sodium hydride in fused caustic soda, excellent; cleaning of various metals may be accomplished at temperatures from 320 to around 600 C. The temperature to be used will depend to some extent on the concentration of hydride, as high concentrations of hydride tend to increase the melting point of the bath. Thus a caustic soda bath having around 15 to 20% by weight of sodium hydride must be operated at around 380 to 400 C. or higher, whereas at a hydride concentration of 1 to 5%, this bath may be operated at about 320 to 350 C.

The hydride concentration of the cleaning bath may vary from 1 to 20% by weight, or even higher. For most purposes, I prefer to maintain a concentration of around 2 to 5% by weight. At the higher concentrations, e. g., around 15%, the possibility of hydride loss due to oxidation is increased and there is some tendency for volatilization of the hydride, indicated by a slight fuming of the bath, especially at the higher temperatures. Because of the reactive nature of the hydride, ordinarily there is no necessity to exceed a hydride concentration of about 5% by weight, nor to operate at temperatures above 350 to 400 C. I prefer to operate at temperatures below 400 C. because at 400 C. or higher the hydride tends to vaporize from the bath.

I have utilized my cleaning process to clean various types of iron and steel, including chrome alloy steels and in each case have found the method effective to remove the commonly occurring surface impurities, leaving a clean, bright surface. Likewise, I have cleaned various non-ferrous metals and alloys, for example, copper, nickel and their alloys, nichrome and various alloys commonly used as electrical resistors. Any metals whose oxides can be reduced by reaction with alkali metal hydride can be readily cleaned by this method. Metal surfaces so cleaned are in excellent condition for electroplating or other operations requiring a perfectly clean surface without further treatment.

An important advantage of my process is that the cleaning bath does not deteriorate with use and may be used practically indefinitely, so long as the required quantity of hydride is added to, or formed in, the bath. The bath does not acmulate compounds of the metals treated, because the surface oxides are reduced to the metallic state. In reacting with the metal oxides, the hydride is converted to'alkali metal hydroxide, a bath constituent. Hence substantially no foreign material accumulates in the bath.

The herein described alkali metal hydride solutions may conveniently be prepared at a distance from the place of utilization, cooled and stored and shipped in sealed containers. On cooling the composition becomes a solid solution of the alkali metal hydride. Thus my invention includes such compositions and is not restricted to the production of those compositions at the time and place of utilization. Furthermore, such compositions are potentially useful for various operations other than cleaning metal surfaces. Since the alkali hydride is an active reducing agent, these compositions may be used in various operations where a highly active reducing agent is required, for example, to reduce metal oxides in order to obtain pure metal therefrom. Furthermore, these compositions are suitable as a source of hydrogen, since they readily react with water to form hydrogen gas. For example, the solid solution of alkali metal hydride may be reacted with cold water in conventional generating apparatus, to obtain substantially complete conversion of the hydride to hydrogen gas and alkali metal hydroxide. For this purpose, my hydride compositions often are more suitable and more convenient to use than other previously known compositions used for generating hydrogen.

My herein described compositions may be used for numerous purposes in place of free alkali metals, for example in chemical reactions and syntheses where sodium and other alkali metals are commonly employed, and various reduction reactions, including reduction of metal oxides. The alkali metal hydrides have been found to react with replaceable hydrogen atoms in organic compounds, for example, with hydroxy compounds. Thus, sodium hydride readily reacts with alcohols to form sodium alcoholates. My hydride compositions may be advantageously employed in place of alkali metals for such purposes, so long as the presence of the alkali metal hydroxide is not disadvantageous.

I claim:

l. The process for removing metal oxide from the surface of a metal article composed of a metal substantially non-reactive with molten alkali metal hydroxides which comprises contacting said artice at a temperature below the melting point of said article with a molten composi-,

tion comprising at least one alkali metal hydroxide containing about 1 to 20% by weight of dissolved alkali metal hydride and then quenching said articles with water.

2. The process for removing metal oxide from the surface of a metal article composed of a metal substantially non-reactive with molten alkali metal hydroxides which comprises contacting said article at a temperature below the melting point of said article with a molten composition comprising sodium hydroxide containing about 1 to 20 per cent by weight of dissolved sodium hydride and then quenching said articles with water.

3. The process for removing metal oxide from the surface of a metal article composed of a metal substantially non-reactive with alkali metal hydroxides which comprises immersing said article in a molten composition comprising at least one alkali metal hydroxide having about i to 20% by weight of alkali metal hydride dissolved therein, at a temperature below the melting point of said article.

4. The process for cleaning ferrous metal ar- 5 ticles to remove metal oxides which comprises immersing said articles in a molten composition comprising sodium hydroxide containing about 1 to by weight of dissolved sodium hydride and then quenching said articles with water.

5. The process for reducing an oxide of a metal substantially nonreactive with molten alkali metal hydroxides to convert said oxide to the corresponding metal which comprises contacting said oxide with a molten composition comprising a1- 5 kali metal hydroxide containing about lto 20 per cent by weight of dissolved alkali metal hydride.

6. The process for reducing an oxide of a metal substantially nonreactive with molten alkali metal hydroxides to convert said oxide to the corre- 20 sponding metal which comprises contacting said oxide with a molten composition comprising sodium hydroxide containing, about 1 to 20 per cent by weight of dissolved sodium hydride.

7. The process which comprises floating a layer of molten alkali metal on the surface of a melt comprising at least one alkali metal hydroxide, passing hydrogen into said melt and into contact with the interface between said alkali metal layer and said melt in an amount sufficient to form a solution of 1 to 20% by Weight of alkali metal hydride in said melt.

8. The process which comprises floating a layer of molten sodium on the surface of a melt comprising sodium hydroxide, passing hydrogen into said melt and into contact with the interface between said sodium layer and said melt in an amount sumcient to form a solution of l to 20 per cent by weight of sodium hydride in said melt.

9. The process for producing a solution of alkali metal hydride in fused alkali metal hydroxide which comprises introducing hydrogen into a fused mass comprising alkali metal monoxide and at least one alkali metal hydroxide, the amount of said monoxide in said mass and the amount of hydrogen introduced therein being sufficient to produce therein a solution of 1 to 20 per cent by weight of said hydride in said hydroxide.

10. The process which comprises passing hy- -drogen into contact with a fused mixture comprising sodium hydroxide and sodium monoxide to convert substantially all of said monoxide into sodium hydride. the amount of said monoxide in the mixture being sufficient to produce a solution of 1 to 20 per cent by weight of said hydride in said hydroxide.

11. A fused bath for removing metal oxide from metal articles composed of metals substantially non-reactive with molten alkali metal hydroxides which'comprises molten alkali metal hydroxide 30 having about 1 to 20% by weight of alkali metal hydride dissolved therein.

12. A fused bath for removing metal oxide from metal articles composed of metals substantially non-reactive with molten alkali metal hydroxides G5 which comprises molten sodium hydroxide having about 1 to 20% by weight of sodium hydrlde dissolved therein.

HARVEY N. GHJEERT. 

